LiMPO4 (wherein, M represents a metal such as Fe or Mn), which is an olivine-type of lithium metal phosphate, is less expensive than LiCoO2, which has conventionally been widely used as a positive electrode active material of lithium secondary batteries. This material is expected to be used in the future as a positive electrode active material of lithium secondary batteries, and particularly large-sized lithium secondary batteries for automotive use.
As described in the prior art documents indicated below, known examples of methods used to produce LiMPO4 include solid-phase synthesis, hydrothermal synthesis and sol-gel methods. Among these, hydrothermal synthesis is superior since it allows the obtaining of LiMPO4 having a small particle diameter at a comparatively low temperature and in a short period of time.
In conventional methods used to carry out hydrothermal synthesis, lithium hydroxide, phosphoric acid and an M element-containing compound are mixed followed by the further addition of water to obtain a raw material, followed by carrying out hydrothermal synthesis by heating this raw material to a temperature of 100° C. or higher in an autoclave. At this time, the particle diameter of the LiMPO4 decreases the smaller the amount of water present.